1. Field of the Invention
The present invention relates to reference voltage supplies for complementary metal-oxide semiconductor (CMOS) circuitry, and more particularly, to a bandgap reference circuit having a bootstrap current generator circuit providing rapid initialization of the bandgap reference circuit.
2. Description of Related Art
Bandgap reference circuits are well known in the art of analog integrated circuit (IC) design for generating a reference voltage equal to the bandgap potential of silicon devices, which is approximately 1.2 volts. Conventional bandgap reference circuits are known to have two stable operating states as the power supply voltage to the circuit transitions from zero to a normal operational level. The first operating state corresponds to a desired mode of the circuit in supplying the reference voltage. The second operating state corresponds to an undesired mode of the circuit in which it remains shut down. A drawback of conventional bandgap reference circuits is that they tend to remain locked-up at the second operating state for an indeterminate period of time before transitioning to the first operating state. It is therefore desirable for many integrated circuit applications that the bandgap reference circuit transition to the first operating state as quickly as possible.
Various techniques are known for speeding the transition to the first operating state. One such technique is to inject a small amount of bootstrap current into the internal nodes of the circuit to prevent it from locking up in the undesired second operating state. For bandgap reference circuits that are supplied by a 3-5 volt power supply, the circuit can include additional transistors that cause the bootstrap current to be injected and then shut off once the first operating state is reached. Unfortunately, this technique is not effective with bandgap reference circuits supplied by a low voltage power supply (e.g., 1-1.5 volts), since the additional transistors in the reference circuit tend to prevent it from turning on. Another solution is to add a resistor leakage path into an internal node of the reference circuit. For low power operation, however, this resistor must be very large (e.g., greater than 10M xcexa9) and it also disturbs the operation of the reference circuit.
Accordingly, it would be very desirable to provide a bandgap reference circuit that overcomes these and other drawbacks of the prior art. More specifically, it would be desirable to provide a bandgap reference circuit that can generate bootstrap current for a power supply voltage ranging between 1-1.5 volts, and that can operate with very low power levels.
The present invention satisfies the need for a bandgap reference circuit that can transition quickly to a desired operational state by injecting bootstrap current into an internal node of the circuit. Unlike the prior art, the present bandgap reference circuit is effective with a low voltage power supply (e.g., 1-1.5 volts).
In accordance with an embodiment of the invention, a bandgap reference generator includes a bandgap reference circuit, a sensing circuit, and a current injector circuit. The bandgap reference circuit includes an input for receiving a supply voltage VCC and an output for providing a reference voltage VREF. The bandgap reference circuit also has a first internal node with a first voltage and a second internal node with a second voltage. The sensing circuit is operatively coupled to the bandgap reference circuit for sensing the second voltage at the second node. The current injection circuit is responsive to the sensing circuit for injecting bootstrap current into the first internal node until the second voltage reaches a threshold voltage. When the second voltage reaches the threshold voltage, reflecting that the desired operating state is achieved, the bootstrap current is shut off.
More particularly, the bandgap reference circuit further includes two n-channel field effect transistors (NFETs) and two p-channel field effect transistors (PFETs). The two NFETs have respective gate terminals tied together at the first internal node and the two PFETs have respective gate terminals tied together at the second internal node. The sensing circuit further comprises a serial pair of inverter circuits adapted to change state at the threshold voltage. The current injection circuit further comprises a depletion field effect transistor (FET) having a source terminal connected to the output of the sensing circuit, and first and second p-channel field effect transistors (PFETs) having respective gate terminals tied together at a drain terminal of the depletion FET. A drain terminal of the second PFET is connected to the first internal node to provide the bootstrap current.
A more complete understanding of a low-voltage, low-power bandgap reference circuit with bootstrap current will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings, which will first be described briefly.